In the use of radiation detector arrays, particularly those referred to as "linear arrays" consisting of a plurality of radiation detectors such as CCD's or CID's arranged in a line, it is often difficult to determine the part of a remote object from which the detector array receives radiation. In photography, the problems is not severe since the human operator has a viewfinder which shows, at least generally, where the detector array is looking. But in many cases, such as assembly line monitoring, the system does not employ a viewfinder or a human operator is not always available to aim through a viewfinder. This problem is quite serious when the equipment must be accurately aimed at a predetermined area on the object. For example, a system might be set up to measure the width of object passing by on a conveyor belt. The object image would fall on the linear array and the number of detector elements covered by the image would then be a measure of the width. Unless the optical system is aimed quite accurately, the image of the object might extend beyond one end of the array and the system would fail. A second problem is also encountered when the system is not in accurate focus because the image on the detectors is fuzzy and the ends of the image are difficult to accurately locate. Of course, increasing the number of detector elements in the array could minimize the problem of the image extending beyond the ends of the array, but the cost of detector elements is high and this solution would still not solve the problem of focus error.